I have today a i7 2600k mounted in my computer(8GB ram, GTX570 OC) and it works great in Battlefield3. Some friends have however said that I should switch to the i5 2500k and get 73,65EURback? This is no performance loss what so ever between i5 2600 and the i7 2600 in games?

Is this true? And is there anything in the future that sugest that I should stick with the i7?

For games, you won't notice a performance difference really as most games use at most 4 cores (and i think that's only BF3). Most 1-2, meaning having 8 (4 physical + 4 from hyperthreading) won't make a difference as they aren't/won't be used at all. The added threads are mainly for programs that use a lot at once, for instance Fold@home, or say handbrake.

Its in swedish but just look at the graphs (green=average red=minimum)

When running the game in 1280x1024 Medium we can see that the CPU does make diffrence(33 fps). When running at 1920x1200 High we can see that there is no diffrece and this is probably due to the limit of the GPU.

So If we think that we will have this CPU a long time and switch at least 2-3 graphic cards before the CPU is to be upgraded. Then we would probably come to a point where the CPU will be the limit and with a i7 we will get alot more fps from then the i5.

Games won't utilize more then 4 cores properly for quite some time, And even at that point when the CPU starts to become a bottleneck with the right cooling you should be able to reach 4.5Ghz or even 5 to overcome that bottleneck.

If you're playing at that low of a resolution you are essentially shifting a lot of the load to the cpu rather then the graphics cards. At a standard gaming resolution (i'd consider 1080p the standard now). You can see it makes no difference whatso ever really. This is going to be the trend for a while until we get more games moving towards better/more multithreading (which if we look at the market now is going to be a long while).